1. Field of the Invention
The present invention relates in general to hydraulic control systems of automatic transmission and more particularly to line pressure control devices therefor.
2. Description of the Prior Art
An example of a prior art line pressure control device is disclosed in the Japanese Provisional Patent Publication No. 59-77155. With the prior art device, line pressure is controlled based on throttle pressure and transmission gear ratio, while the throttle pressure is controlled based on intake manifold vacuum (engine load), i.e., the throttle pressure decreases with decreasing intake manifold vacuum, while the line pressure is controlled so as not to become smaller than a predetermined minimum value. The line pressure, after decreasing to the predetermined minimum value, is thus held constant irrespective of further increase of the intake manifold vacuum. Such a line pressure variation characteristic is exemplarily shown in the graph of FIG. 14.
Referring to FIG. 14, engine torque varies directly with the intake manifold vacuum as indicated by the solid line, i.e., the engine torque increases to a maximum value when the intake manifold vacuum decreases to zero (or when the intake manifold pressure decreases to a certain positive value in the case of a turbo-charger being employed) and decreases to zero when the intake manifold vacuum increases to a certain value, e.g. 400 mmHg as shown in the graph of FIG. 14. Under engine brake conditions (i.e. under driving conditions where engine output torque is negative), the intake manifold vacuum increases further. Even under such engine brake conditions, the clutches of the transmission or pulleys in the case of the continuously variable V-belt type are needed to have such torque transmission capacities that match the engine brake torque, i.e., needed to be able to transmit such torque that is represented by the dotted line in the graph of FIG. 14. The dotted line and the negative part of the engine torque line are symmetrical with respect to the axis of absccisa. For this reason, the throttle pressure and the line pressure corresponding to the intake manifold vacuum of around 400 mmHg become larger than needed. Since the intake manifold vacuum range of around 400 mmHg is most frequently used under ordinary driving, the line pressure higher than needed under that vacuum range inevitably results in an increased loss of the oil pump and therefore a lowered efficiency of the transmission. This is particularly true when the line pressure control device is used in a continuously variable V-belt transmission.